Unit 2: Lasers and Applications - Practice Quiz

1 What does the acronym LASER stand for?

2 Which statement correctly describes the energy levels of an atom?

3 Which process occurs when matter interacts with electromagnetic radiation?

4 During absorption, an atom moves from a lower energy level to a higher energy level . What is the required photon energy?

5 How are photons emitted during spontaneous emission?

6 A photon produced by stimulated emission has the same frequency, phase, and direction as which photon?

7 Under normal thermal equilibrium, which energy level usually contains the greatest number of atoms?

8 The Einstein coefficient represents the probability of which process?

9 What is the main characteristic of a metastable state?

10 When does population inversion occur between two laser energy levels?

11 What is the main purpose of the mirrors in a laser resonant cavity?

12 What is the process of supplying energy to the active medium of a laser called?

13 Which emission process is primarily responsible for light amplification in a laser?

14 Which property means that laser light has a very narrow range of wavelengths?

15 Which material acts as the active medium in a ruby laser?

16 Which ion provides the laser action in an Nd-YAG laser?

17 What is the common visible output wavelength of a He-Ne laser?

18 Which structure is commonly used as the active region of a semiconductor laser?

19 Which engineering operation commonly uses a high-power laser?

20 What information about an object wave is recorded in holography?

21 Which combination is essential for sustained laser operation?

22 An atom makes a transition from an energy level at to one at . What is the approximate wavelength of the emitted photon?

23 For a two-level atomic system exposed to radiation of energy density , which expression represents the total downward transition rate per unit volume?

24 What is the approximate energy absorbed by an atom when it absorbs a photon of wavelength ?

25 An excited state has a mean lifetime of . Assuming exponential decay, what fraction of atoms will have emitted spontaneously after ?

26 A photon stimulates an excited atom to emit another photon. Which property must the emitted photon have relative to the incident photon?

27 Two nondegenerate energy levels are separated by . At , what is the approximate equilibrium ratio ? Use .

28 For two nondegenerate levels in thermal equilibrium, which relation between the Einstein coefficients is correct?

29 Why is a metastable state useful in a laser active medium?

30 For a laser transition between levels and having equal degeneracies, which condition produces optical gain?

31 A laser cavity has length and refractive index . What is the longitudinal mode spacing?

32 Which pairing of a laser and its commonly used excitation mechanism is correct?

33 A laser medium has a small-signal gain coefficient of and distributed losses of . What happens to light during one pass through the medium before gain saturation?

34 A laser beam has a divergence of and an initial diameter of . Neglecting diffraction details, what is its approximate diameter after traveling if divergence denotes the full angular spread?

35 Why does a ruby laser generally require intense pumping and often operate in pulsed mode?

36 Which statement best explains why population inversion is easier to maintain in an Nd-YAG laser than in a ruby laser?

37 What is the principal role of helium atoms in a He-Ne laser?

38 A direct-bandgap semiconductor has a bandgap of . What is the approximate wavelength of its emitted laser light?

39 An engineer must measure a very small displacement of a machine component without physical contact. Which laser-based method is most suitable?

40 During hologram recording, why is a reference beam combined with the light scattered from the object?

41 A laser cavity of length has mirror reflectivities and , with distributed internal loss . Assuming uniform gain, what is the threshold gain coefficient?

42 Two atomic levels in thermal equilibrium have and degeneracies . What is at , using ?

43 Levels 1 and 2 have degeneracies and . For radiation resonant with the transition, which population condition produces transparency when spontaneous emission is neglected?

44 For a transition with stimulated-emission cross-section and degeneracies and , which expression represents the net resonant absorption coefficient, assuming identical normalized line shapes?

45 An excited state decays radiatively through two channels having Einstein coefficients and . What are its radiative lifetime and the branching probability for emission to level 1?

46 An atom is in thermal radiation for which . What is the ratio of the stimulated-emission rate to the spontaneous-emission rate for the transition?

47 A nondegenerate two-level atomic system is in thermal equilibrium at a positive absolute temperature. Which statement about its level populations is necessarily true?

48 If is blackbody spectral energy density per unit ordinary frequency, which Einstein relation follows from equilibrium with Planck's distribution?

49 In an idealized three-level laser, pumping transfers atoms from the ground state rapidly into a metastable state. If the effective pump rate per ground-state atom is and the metastable lifetime is , what condition gives population inversion between these two states?

50 A fraction of the active atoms is in the upper laser level. Neglecting other populated states, what are the inversions for an ideal three-level laser and an ideal four-level laser, respectively?

51 A linear cavity has and refractive index . A gain band of full width is centered exactly on one longitudinal mode. How many longitudinal modes lie within the band?

52 An Nd:YAG laser is pumped at and emits at . Assuming one laser photon can result from each absorbed pump photon, what is the maximum energy-conversion efficiency imposed solely by the quantum defect?

53 A homogeneously broadened laser has saturated gain . If , what intracavity intensity is expected in steady-state operation above threshold?

54 A laser has a Lorentzian spectral line with full width at half maximum . If first-order coherence decays as , what is its coherence length in vacuum?

55 In a simplified ruby laser, only the ground and metastable laser levels retain significant populations. If the threshold inversion is , what minimum fraction of chromium ions must occupy the metastable state?

56 In a simplified four-level Nd:YAG model, atoms flow through the upper and lower laser levels at the same steady rate. If their lifetimes are and , respectively, what is ?

57 Which sequence most accurately describes production of the output in a He-Ne laser?

58 A semiconductor laser emits near and has cavity length with group index . What is the approximate wavelength spacing of adjacent longitudinal modes?

59 In a Michelson interferometer using a laser, 500 fringes cross the detector when one mirror is displaced. What is the mirror displacement?

60 A transmission hologram is recorded at using a normally incident object wave and a reference wave at . It is then illuminated normally at . At what approximate angle does the corresponding first-order reconstructed wave emerge?